This invention relates to an implantable positive pressure infusion reservoir. In particular, it relates to an implantable drug infusion reservoir that eliminates the requirement for a bellows and two-phase propellent used as a primary drug storage reservoir.
Implantable drug infusion systems are commercially available. Typical are the Infusaid Model 100 and 400 pump systems. Those systems are based on the technology disclosed in U.S. Pat. No. 3,731,681 which employs a bellows, typically made of titanium for purposes of drug compatibility, as the main drug storage reservoir. Titanium has been chosen in the prior art additionally for its biocompatibility and its hermeticity when welded to other components of the pump. Moreover, systems in use today predicated on the technology of the '681 patent typically use Freon (DuPont trademark) as a two-phase constant pressure media for purposes of providing a delivery pressure tending to bias the bellows in a compressed position.
While such systems have become commercially viable, they are relatively expensive. For example, the titanium bellows per se typically costs between $100-$200. This is the piece part cost and does not include the costs associated with welding, Freon backfill and the like. Thus, the use of a bellows/Freon system and its corresponding material requirements are a major contributing factor in the cost of an implantable drug infusion system.
Additionally, the use of a bellows configuration carries with it inherent size and shape limitations and becomes less volume efficient for bellows having small diameters, long strokes and low spring rate pressures. To date, however, the use of the bellows configuration is the only major commercially viable system.
Other techniques have been proposed in the art such as U.S. Pat. No. 3,840,009. This reservoir concept employs a heat-sealed metallized film bag for containment of a two-phase charging fluid. The bag is then immersed in the drug solution and acts as a compliant reservoir as well as a motive pressure force. In such a system then, the use of the two-phase fluid is also required with the propellent media housed in a flexible member inside a larger volume reservoir.
Such a configuration, while eliminating the bellows may still be expensive to manufacture since environmental concerns have decreased the availability and raised the cost to purchase and store chlorofluorocarbons of the type used for implantable propellents.
Thus, there exists within the technology dealing with implantable drug infusion systems the need for a lower cost reservoir which utilizes less parts yet does not decrease the volumetric efficiency of traditional bellows devices. Moreover, such a system must be compatible with the stringent requirements of implantation into a living body together with drug compatibility over a range of drugs to be infused.